Genetic approaches to three experimental systems of decreasing morphogenetic complexity are proposed. Genetic control of the yeast cell division cycle will be investigated using cold-sensitive (cs) and heat-sensitive (ts) mutations. The reciprocal temperature-shift method for determining order of events using cs/ts double mutants will be applied. Isolation of new cell-cyle mutations by reversion of old ones will be continued. Bacterial DNA replication will be investigated using similar genetic methods. Cold-sensitive and heat-sensitive mutants of Salmonella typhimurium in DNA replication will be isolated and characterized. Mapping and complementation studies (within Salmonella and in comparison with E. coli) will be undertaken using powerful new methods of genetic manipulation in vivo which take advantage of the properties of translocatable drug-resistance elements. Pseudo-reversion will be used to find new DNA replication mutants, as will localized mutagenesis. Episomes carrying particular regions of the Salmonella chromosome will be constructed in vivo. In order to learn more about the relationship of protein structure to function, saturation genetics of the structural gene for the TEM-2 beta-lactamase will be undertaken. The gene is carried on phage P22 carrying the translocatable element Tn1. Nonsense, frameshift, missense (including cs and ts) mutants will be isolated and mapped by deletion mapping. The detailed phenotypes of these mutations will be assessed, and interesting cases analyzed crystallographically in another laboratory.